CIO - who has done it and what happened with your child?

yep, did CIO with my oldest stepson (who is now 13). It was horrible, he'd not give up and sometimes he'd make himself throw up :( It definitely caused some sleep issues with him... for A LONG time he needed someone in the bed with him until he fell asleep and would stress & worry at bedtime. I followed Elizabeth Pantley's No Cry Sleep Solution book with my 7yo and his bedtime routine went so much better, he's a good sleeper now, no issues at all (he puts himself to bed!).

I used a modified CIO with our DS. It worked pretty well, but did NOT make him STTN, but did help him be able to get himself back to sleep without my intervention if/when he woke at night. He didn't start STTN until a few months ago (he's now 22 months). However, it did greatly help. Here's basically what we did:
1) come up with a bedtime routine and stick to it EVERY SINGLE NIGHT
2) put him down awake (also, do not feed him in his room as he can associate food with sleep even if not falling asleep while eating/drinking)
3) leave the room
4) if he cries, wait "x" amount of time before going back in
a) you can pick any starting amount that you want...I think I did 1-2 minutes at first
5) go back in and comfort him, but DO NOT pick him up: rub his back, "shush" anything but pick him up (although if he is hysterical, I would pick him up) and only stay in there for 1-2 minutes max
6) leave the room again
7) if he cries again, go back in after "y" minutes
a) I think we went with 5 minutes for the second time
8) comfort him again as above and then leave
9) if he still cries, go back in after "z" minutes and do the comforting thing again and then leave
a) I think we did 7 minutes at this point
10) For each time you have to go back in after the third time, use the same amount of time...so for the first night, the max time between going to comfort would be something like 7 minutes
11) do this for several nights, and then increase the amount of time between comfortings by a little...do that for a few nights and then increase again, etc.

He *should* eventually get himself to sleep. That is the most important part of CIO: teaching them to get themselves to sleep and breaking any sleep associations they may have developed (bottle, rocking, being held, etc.). Just know that CIO does not work for every child, but it does generally work.

My DS didn't become clingy during the day when we did CIO. And yes, it killed me to let him cry, even for a minute or two, but I was so exhausted from getting up 4, 5, 6 or more times a night with him that I had to do something. We actually got our instructions from a child sleep psychologist because I was desperate for some sleep for myself! I can imagine if you are leaving him to cry for LONG periods of time, that he may become more needy/clingy during the day. But if you go to him periodically during the CIO, then he should be reassured that you are still there for him, and not be as clingy during the day. Also, remember that it may be something else that is causing him to be clingy, like teething.

We did a modified CIO when DD was about 8-9 months old. We co-slept up until then. It was a rough couple weeks, but she then comforted herself. Since doing that. She has been a GREAT sleeper!! no issues at all unless she was going through a growth spurt or was sick! I will be doing it again with this one for sure.